1. Field of the Invention
The present invention relates to a method for measuring the epitaxial film thickness of multilayer epitaxial wafer, and particularly to a method for measuring the epitaxial film thickness of two-layer epitaxial wafer for IGBT (Insulated Gate Bipolar Transistor) for use in a power device such as a switching element or the like under a non-destructive and non-contact state with high precision.
2. Description of Related Art
Epitaxial wafers having such a multilayer structure that epitaxial thin films having different electrical characteristics (hereinafter referred to as "epitaxial wafer") are formed in a multilayer structure (hereinafter referred to as "epitaxial wafer") such as IGBT wafer or the like have been increasingly applied to power devices, and much attention has been paid to such epitaxial wafers. The epitaxial film thickness is also an important factor to obtain devices having excellent characteristics from these multilayer-structure epitaxial wafers, so that it has been required to control the epitaxial film thickness with high precision in a manufacturing process. The control of the epitaxial film thickness first necessitates measurement of the thickness of an epitaxial film. The measurement of the thickness of an epitaxial film which is formed by inducing epitaxial growth on a silicon monocrystal substrate has been hitherto carried out in consideration of the relation with device characteristics. According to a conventional film thickness measurement method, the spreading resistance on a oblique polished surface of an epitaxial wafer is measured, that is, the conventional measurement is a destructive measurement. Accordingly, it has a problem in sampling, and thus a non-destructive measurement has been required.
In such a condition as described above, much attention has been paid to a film thickness measurement based on a middle infrared reflection spectroscopy which generally uses FTIR (Fourier Transform Infrared Spectrophotometer), and various applications thereto have been tried. According to the film thickness measurement using the FTIR method, a reflection spectrum of an epitaxial wafer is measured by using an infrared spectrophotometer, and an interference fringe pattern on the spectrum is obtained to analyze the interference fringe pattern. With this measurement, the film thickness can be measured on the basis of the interference fringe pattern by relatively simple analysis when the thin film formed on the substrate is a single layer. For example, the epitaxial film thickness of the single-layer epitaxial wafer can be detected by irradiating to a sample infrared ray of a middle infrared region of 4000 to 400 cm.sup.-1 (in wavenumber) and then measuring the interference fringe between reflected light at the interface between the substrate and the epitaxial layer (hereinafter referred to as "epitaxial layer/substrate interface") and reflected light at the surface of the epitaxial wafer. In general, the substrate of the epitaxial wafer has a high carrier concentration of 10.sup.18 to 10.sup.19 cm.sup.-3 level. On the other hand, the carrier concentration of the epitaxial layer is lower than the substrate. Therefore, the incident infrared ray is transmitted through the epitaxial film layer of low concentration, reflected at the epitaxial-layer/substrate interface and then interferes with the reflected light at the surface of the wafer, so that interference fringe is observed on the reflection spectrum. That is, the interference fringe is formed due to the difference in optical length between the reflected light at the epitaxial-layer/substrate interface and the reflected light at the wafer surface, and the epitaxial film thickness is measured on the basis of the relationship in which the period of the interference fringe is inversely proportional to the epitaxial film thickness.
On the other hand, with respect to a multilayer film, reflected light at the interface between respective epitaxial layers other than the reflected light at the epitaxial-layer/substrate interference also contributes to the interference fringe pattern. Accordingly, the interference pattern thus obtained is complicated and it is very difficult to measure the film thickness by directly performing waveform analysis of the interference pattern. Therefore, there has been proposed a method of measuring the thickness of each layer of the multilayer-structure film. For example, with respect to compound semiconductors such as GaAs group, etc., there has been proposed a method in which the epitaxial film thickness of each layer of the multilayer epitaxial film structure is measured by performing the reflection measurement in the range from the near infrared region to the middle infrared region in the same manner as the single-layer epitaxial layer, and then subjecting the reflection measurement result to Fourier Transform. That is, according to this method, by using the FTIR method, light in the wavenumber region from the visible area to the infrared region is irradiated to a compound semiconductor having a multilayer film structure to obtain a reflection spectrum, film interference components which are obtained from the reflection spectrum are subjected to inverse Fourier Transform to obtain a so-called spatialgram, and then the multilayer film thickness is measured on the basis of the waveform analysis of the spatialgram ("Electronic Materials" Vol.28 (No. 11), pp40-45 (1989), "Optics" Vol.20, pp305-309 (1991)). Further, with respect to the measurement of the multilayer film thickness, Japanese Laid-open Patent Application NO. Hei-5-302816 proposes that the measurable wavenumber range is extended by using a plurality of photodetectors, light-transmissible members or light sources to enhance the measurement limitation of the film thickness, and Japanese Laid-open Patent Application No. Hei-7-4922 proposes that the waveform of a theoretical interference spectrum is calculated on the basis of the measurement value of the film thickness obtained from the spatialgram, and then waveform fitting is performed to thereby measure the film thickness with high precision.
However, according to the middle infrared reflection spectroscopy, it is impossible to measure the epitaxial film thickness of each layer of a multilayer structure epitaxial wafer in which at least similar single crystalline silicon two layers having different electrical characteristics are formed on a single crystalline silicon substrate. However, for an epitaxial wafer having a two-layer structure, the total film thickness of the first and second layers can be detected by measuring the interference fringe between the reflected light at the first layer/substrate interface and the reflected light at the wafer surface.
As shown in FIGS. 16A to 16D (FIG. 16A is a schematic diagram showing a general two-layer structure of an IGBT wafer which is a target of the present invention, FIGS. 16B and 16C are schematic structural diagrams showing two typical kinds of two-layer epitaxial wafers, and FIG. 16D shows infrared ray reflection in the two-layer epitaxial wafer), in the case of the two-layer epitaxial wafer, it is general that two layers having different electrical characteristics, that is, a first layer of 5 to 25 .mu.m in thickness which is doped with phosphorus (P) or boron (B) of high concentration (specific resistance of about 30 to 100 m.OMEGA.cm) and a second layer of 30 to 130 .mu.m in thickness which is doped with phosphorus (P) or boron (B) of low concentration (specific resistance of about 10 .OMEGA.cm or more) are continuously epitaxially grown on a single crystalline silicon substrate which is doped with boron or antimony (Sb) of relatively high concentration (specific resistance of about 25 to 20 m.OMEGA.cm or less). In this case, the carrier concentration of the first layer is not sufficiently high, and the difference in carrier concentration between the first and second layers is not sufficiently high, and thus it is generally difficult to measure the interference fringe between the reflected light at the second layer/first layer interface and the reflected light at the wafer surface to measure the film thickness of the second layer, and the interference fringe between the reflected light at the first layer/substrate interface and the reflected light at the second/first interface to measure the film thickness of the second layer.
As described above, it is difficult to measure the epitaxial film thickness of each layer of the multilayer epitaxial wafer by the analysis using the FTIR method, however, it is possible to measure the total film thickness of the overall multilayer epitaxial film thickness. Accordingly, the following methods may be used to determine the epitaxial film thickness of each layer in the manufacturing process. According to the first method, as a test sample, only a first layer is epitaxially grown and then the film thickness thereof is measured as the thickness of the first layer by the FTIR method. Subsequently, a first layer and a second layer are continuously epitaxially grown under the same condition as the case where only the first layer is formed, and the total film thickness of the first layer and the second layer are measured by the FTIR method, and the subtraction of the film thickness between the above two samples is calculated as the film thickness of the second layer. According to the second method, the growth time of each epitaxial film of the first and second layers is measured, and the film thickness of each epitaxial layer is calculated by dividing the total film thickness of the first and second layers by the growth time of each epitaxial layer. These methods enable the non-destructive measurement of the epitaxial film thickness of the first and second layers of products, however, this method is not preferable because the measurement contains some estimation.